U.S. patent application number 16/647544 was filed with the patent office on 2020-08-27 for electrothermal heater mat.
The applicant listed for this patent is GKN Aerospace Services Limited. Invention is credited to Stephen Goodfellow-Jones.
Application Number | 20200269526 16/647544 |
Document ID | / |
Family ID | 1000004866377 |
Filed Date | 2020-08-27 |
United States Patent
Application |
20200269526 |
Kind Code |
A1 |
Goodfellow-Jones; Stephen |
August 27, 2020 |
ELECTROTHERMAL HEATER MAT
Abstract
In manufacturing an electrothermal heater mat, there is provided
a preform which comprises a laminated stack of dielectric layers
which are made of thermoplastic material and include a central
layer or group of layers which include(s) reinforcement and first
and second outer groups of layers which do not include
reinforcement. The preform includes a heater element and the
preform has a first configuration. The preform is then heated to a
temperature (e.g. 180.degree. C.) between the glass-transition
temperature of the thermoplastic material and the melting point of
the thermoplastic material, and the heated preform is formed into a
second configuration which is different to the first configuration
so as to produce the heater mat.
Inventors: |
Goodfellow-Jones; Stephen;
(Redditch, Worcestershire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GKN Aerospace Services Limited |
Redditch, Worcestershire |
|
GB |
|
|
Family ID: |
1000004866377 |
Appl. No.: |
16/647544 |
Filed: |
September 18, 2018 |
PCT Filed: |
September 18, 2018 |
PCT NO: |
PCT/GB2018/052657 |
371 Date: |
March 16, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B64D 15/12 20130101;
H05B 3/36 20130101; B29C 70/885 20130101; B29L 2031/779 20130101;
B29C 70/30 20130101 |
International
Class: |
B29C 70/30 20060101
B29C070/30; B29C 70/88 20060101 B29C070/88; B64D 15/12 20060101
B64D015/12; H05B 3/36 20060101 H05B003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 19, 2017 |
GB |
1715093.9 |
Claims
1.-20. (canceled)
21. A method of manufacturing an electrothermal heater mat,
comprising: providing a preform that includes a laminated stack of
dielectric layers made of thermoplastic material, the stack
including a central group of one or more layers, and first and
second outer groups of layers; wherein the central group includes
reinforcement and the outer groups do not include reinforcement;
wherein the preform includes a heater element and the preform has a
first configuration; heating the preform to a temperature between a
glass-transition temperature of the thermoplastic material and a
melting point of the thermoplastic material; and forming the heated
preform into a second configuration which is different from the
first configuration so as to produce the heater mat.
22. The method of claim 21, wherein the first configuration is a
substantially planar configuration.
23. The method of claim 21, wherein the second configuration is a
curved configuration in which the heater mat is non-planar.
24. The method of claim 21, wherein, in the second configuration,
the heater mat is generally U-shaped in cross-section.
25. The method of claim 21, wherein: in the forming step, the
preform is positioned on a tool which has a tool surface with a
third configuration which is more curved than the second
configuration and the preform is formed down onto the tool surface;
and after the heater mat has been formed on the tool, the heater
mat relaxes from the third configuration to the second
configuration.
26. The method of claim 21, wherein in the heating step the preform
is heated to and held at a temperature in a range of 150.degree. C.
to 210.degree. C.
27. The method of claim 21, wherein in the forming step the heated
preform is held at a temperature for a period of time from 15 to 60
minutes.
28. The method of claim 21, wherein the reinforcement is woven
e-glass.
29. The method of claim 21, wherein in the forming step the heated
preform is positioned on a tool having a tool surface with a ridge
and the preform is formed around the ridge.
30. The method of claim 29, wherein the ridge includes positioning
pins and the preform has apertures which are fitted onto the
pins.
31. The method of claim 21, wherein in the forming step the heated
preform is positioned on a tool having a convex tool surface and a
vacuum bag is used to hold the preform down onto the convex tool
surface.
32. The method of claim 21, wherein: in the forming step the heated
preform is positioned on a tool having a convex tool surface; and
after the forming step the heater mat produced by forming the
preform is returned to an ambient temperature before the heater mat
is removed from the tool surface.
33. The method of claim 21, wherein the second configuration is
generally U-shaped in a transverse direction of the heater mat and
tapers and/or twists in a longitudinal direction of the heater
mat.
34. The method of claim 33, wherein the preform is formed against a
tool which has a tool surface which is convex in a transverse
direction and which become more convex in a longitudinal direction
and/or twists in a longitudinal direction.
35. The method of claim 21, wherein, in the heating and forming
steps, the method further comprises positioning the preform against
a component including thermosetting material and using heating of
the heating step to bond the preform to the component.
36. A method of manufacturing an electrothermal heater mat,
comprising: laying up a stack comprising substantially planar
layers which include a plurality of dielectric layers and a heater
element layer; wherein each layer comprises thermoplastic material;
heating the stack to a first temperature and laminating together
the layers of the stack so as to produce a preform; positioning the
preform on a convex surface of a tool; heating the preform to a
second temperature which is lower than the first temperature and is
in a range of 150.degree. C. to 210.degree. C.; and forming the
preform around the convex surface of the tool.
37. The method of claim 36, wherein, in the heating and forming
steps, the method further comprises positioning the preform against
a component including thermosetting material and using heating of
the heating step to bond the preform to the component.
38. The method of claim 37, wherein the component has a surface
which has the second configuration and to which the preform is
bonded.
39. The method of claim 37, wherein, in the heating and forming
steps, the preform is positioned between a tool and the component.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a national stage of, and claims priority
to, Patent Cooperation Treaty Application No. PCT/GB2018/052657,
filed on Sep. 18, 2018, which application claims priority to Great
Britain Application No. GB 1715093.9, filed on Sep. 19, 2017, which
applications are hereby incorporated herein by reference in their
entireties.
BACKGROUND
[0002] For an aircraft, the in-flight formation of ice on the
external surface of the aircraft is undesirable. The ice hinders
the smooth flow of air over the aircraft surface, increasing drag
and additionally decreasing the ability of an aerofoil to perform
its intended function.
[0003] Also, built-up ice may impede the movement of a movable
control surface such as a wing slat or flap. Ice which has built up
on an engine air inlet may be suddenly shed in large chunks which
are ingested into the engine and cause damage.
[0004] It is therefore common for aircraft, and particularly
commercial aircraft, to incorporate an ice protection system. A
commercial aircraft may use a system which involves bleeding hot
air off from the engines, and the hot air is then ducted to the
airframe components such as the leading edges of the wing and the
tail which are prone to ice formation. More recently, electrically
powered systems have been proposed, such as in EP-A-1,757,519 (GKN
Aerospace) which discloses a wing slat having a nose skin which
incorporates an electrothermal heater blanket or mat. The heater
mat is bonded to the rear surface of a metallic erosion shield
which comprises the forwardly-facing external surface of the nose
skin.
[0005] The heater mat is of the SPRAYMAT (trade mark) type and is a
laminated product comprising dielectric layers made of
pre-impregnated glass fibre cloth and a heater element formed by
flame spraying a metal layer onto one of the dielectric layers. The
"Spraymat" has a long history from its original development in the
1950s by D. Napier & Sons Limited (see their GB-833,675
relating to electrical de-icing or anti-icing apparatus for an
aircraft) through to its subsequent use by GKN Aerospace.
[0006] A "Spraymat" produced in recent years for use in a wing slat
is formed on a male tool and involves laying up a stack of layers
comprising (i) about 10 layers of glass fibre fabric
pre-impregnated with epoxy which has been cured in an autoclave,
(ii) a conductive metal layer (the heater element) which, as has
been done in previous products, has been flame sprayed onto the
laminate using a mask to form the heater element pattern and (iii)
a final 3 or so layers of the glass fibre fabric. Wiring is
soldered to the heater element to permit connection to the
aircraft's power system. The heater mat is then cured in an
autoclave.
[0007] A heater mat often incorporates a conductive ground plane as
a safety device for detecting a fault with a heater element of the
heater mat. The ground plane is connected to an aircraft earth as
well as to a control unit.
[0008] A heater mat is generally very reliable. However, if the
heater element in the heater mat does develop a fault in the form
of heater burn-out, current will leak to the aircraft earth via the
ground plane and the control unit can detect this change in current
and take action to prevent thermal damage to the structure of the
heater mat.
[0009] GKN Aerospace has recently developed a technique of applying
the heater element and the ground plane as a flame sprayed metal
layer (such as of copper or copper alloy) where the heater element
or ground plane is sprayed (using a mask) onto a dielectric ply
layer which is made of thermoplastic material instead of the
previously-used thermosetting (e.g. epoxy resin) material. This
newer type of arrangement for the heater element and ground plane
is described in GB-A-2,477,336, GB-A-2,477,337, GB-A-2,477,338,
GB-A-2,477,339, and GB-A-2,477,340 (all in the name of GKN
Aerospace), the disclosures of which are incorporated herein by
reference in their entireties.
[0010] Glass-reinforced thermoplastic laminates are widely used in
the aerospace and automotive industries when thermoplastic material
is being used as an alternative to epoxy-glass (thermosetting)
material.
[0011] However, engineering thermoplastics usually have higher
melting points relative to epoxy materials, and the higher
processing temperatures that are involved require higher investment
in capital equipment, tooling and consumables.
[0012] For example, thermoplastic typically needs to be heated to
about 400.degree. C. when fusing laminate layers together. This
means that the tool on which the lamination is performed needs to
be made of steel instead of the aluminium typically used for the
lower processing temperatures of thermosetting materials.
[0013] When producing a heater mat which is required to have a 3-D
(curved) shape or configuration, the desired final shape of the
heater mat may be achieved directly by performing the lamination on
a steel tool having the desired 3-D shape.
[0014] Alternatively, the layers of the heater mat may initially be
laminated together whilst the heater mat has a flat (2-D) shape, on
a flat table made of steel. By performing the initial (first) step
of the production on a flat table, a high production rate may be
achieved. The heater mat is then formed into the desired final,
curved (3-D) shape using tooling, for example by bending (by
forming) the 2-D heater mat around a suitably curved (3-D) tool
surface of a tool. The tool for this second step is made of steel
because the preform (the 2-D heater mat) is re-heated to above the
melting point of the thermoplastic material of the preform, to a
temperature which may affect the tempering and shape of an
aluminium tool. During this second (forming) step, a vacuum bag
typically made of polyimide material covers the preform to assist
with the forming operation being performed on the heater mat.
[0015] It would be desirable to improve the production of a heater
mat made of thermoplastic composite.
SUMMARY
[0016] The present disclosure relates generally to an
electrothermal heater mat and method of manufacture thereof,
including an electrothermal heater mat for use in an ice protection
system. The electrothermal heater mats are suitable for a range of
applications where ice may form on a surface but in particular they
are suitable for use in an aircraft or other aerodynamic structure
such as a blade of a wind turbine to prevent ice from forming
and/or to remove ice that has already formed. These two functions
may be termed anti-icing and de-icing, respectively.
[0017] According to an aspect, there is provided a method of
manufacturing an electrothermal heater mat, comprising the steps
of: providing a preform which comprises a laminated stack of
dielectric layers which are made of thermoplastic material and
include a central layer or group of layers which include(s) a
reinforcement material and first and second outer groups of layers
which do not include the reinforcement material; wherein the
preform includes a heater element and the preform has a first
configuration; heating the preform to a temperature between the
glass-transition temperature of the thermoplastic material and the
melting point of the thermoplastic material; and forming the heated
preform into a second configuration which is different to the first
configuration so as to produce the heater mat.
[0018] Because the forming is being performed at a temperature
below the melting point of the thermoplastic material, the
temperature associated with the forming step is significantly lower
than was previously the case when forming heater mats made of
thermoplastic composite, and thus the tooling that is used does not
have to resist the previous high temperatures. Therefore, instead
of having to use heavy steel tooling, it is possible to use the
standard aluminium tooling typically used for forming epoxy
(thermosetting) composites. This may produce a significant cost
saving.
[0019] The reinforcement is limited to being at or predominantly at
the central thickness of the preform (effectively at the neutral
axis for the bending associated with the forming step) and this
enables the forming to occur despite the fact that the matrix
material of the preform (the thermoplastic) is only being softened
and not melted.
[0020] The thermoplastic matrix undergoes a form of creep when
softened, and this enables the heater mat preform to be shaped in
the forming step.
[0021] The lower temperature associated with only softening the
thermoplastic matrix material means that the electrical
characteristics of the heater element are less impaired by the
heating associated with the forming operation. Similarly, if the
preform includes other functional components (e.g. a conductive
ground plane), they also experience reduced impairment from the
heating than would be the case with a higher temperature associated
with actually melting the thermoplastic matrix material.
[0022] The thermoplastic material may be PEEK, PEKK, PPS, PEI or
PES or a mixture thereof. More preferably, the thermoplastic
material is PEEK, PEKK or a mixture thereof.
[0023] When the first configuration of the preform is an un-flexed
configuration (e.g. a flat configuration), the preform in the
forming step may be flexed into the second configuration and
initially it will have a tendency to want to return to the first
configuration. As the forming step progresses, the thermoplastic
material of the heated preform creep-forms, and the permanent shape
(or set) of the preform progresses towards the second
configuration.
[0024] In examples, the first configuration is a substantially
planar configuration, and/or the second configuration is a curved
configuration in which the heater mat is non-planar.
[0025] In the second configuration, the heater mat may have a
concave rear face and a convex front face. This will often be the
case when the heater mat is intended to be used on an aerofoil
leading edge.
[0026] In examples, all of the dielectric layers of the preform are
made from thermoplastic material. The same thermoplastic material
may be used for each dielectric layer of the preform.
[0027] In some examples, the preform includes one or more
conductive ground plane layer(s) and/or a temperature sensor
layer.
[0028] In some examples, the method may include the preliminary
steps of:
[0029] providing a stack containing a plurality of dielectric
layers and a heater element layer;
[0030] wherein each layer comprises thermoplastic material; and
[0031] heating the stack to a temperature above the melting point
of the thermoplastic material and laminating together the layers of
the stack so as to produce an intermediate product (the preform)
having the first configuration.
[0032] In some examples, in the second configuration, the heater
mat is generally U-shaped in cross-section.
[0033] In some examples, in the forming step, the preform is
positioned on a tool which has a tool surface with a third
configuration which is more-curved than the second configuration
and the preform is formed down onto the tool surface; and, after
the heater mat is removed from the tool, the heater mat relaxes
from the third configuration to the second configuration. The
degree of "spring back" from the third configuration to the second
configuration will depend on factors such as the duration of the
heating and forming operations and the temperature during the
heating and forming operations.
[0034] In some examples, in the heating step, the preform is heated
to and held at a temperature in the range 180.degree.
C..+-.30.degree. C. (or .+-.20.degree. C., or .+-.10.degree. C.).
Thus, the forming is performed at a temperature which is above the
typical glass-transition temperature of 140 to 150.degree. C. of
the thermoplastic materials that are typically used in the
production of heater mats.
[0035] In some examples, the preform has a thickness of 1 mm or
less. This results in a heater mat of similar thickness, and the
thickness is less than the thickness (of 2 to 3 mm) which is
typical of current epoxy heater mats.
[0036] In some examples, in the forming step, the heated preform is
held at the temperature for a period of time from 15 to 60 minutes
(or from 20 to 40 minutes, or for about 30 minutes).
[0037] In some examples, the reinforcement is glass reinforcement,
e.g. woven e-glass.
[0038] In some examples, in the forming step, the heated preform is
positioned on a tool having a tool surface with a ridge and the
preform is formed around the ridge. The ridge may include
positioning pins and the preform has apertures which are fitted
onto the pins. The apertures may be positioned along a central
longitudinal axis of the preform. The apertures and pins may serve
to achieve a desired relative positioning of the preform on the
tool, and to maintain that positioning during the heating and
forming operations.
[0039] In some examples, in the forming step, the heated preform is
positioned on a tool having a convex tool surface and a vacuum bag
is used to hold the preform down onto the convex tool surface.
[0040] In some examples, in the forming step the heated preform is
positioned on a tool having a convex tool surface; and after the
forming step the heater mat produced by forming the preform is
returned to an ambient temperature before the heater mat is removed
from the tool surface.
[0041] In some examples, the second configuration is generally
U-shaped in a transverse direction of the heater mat and tapers
and/or twists in a longitudinal direction of the heater mat.
[0042] In some examples, the preform is formed against a tool which
has a tool surface which is convex in a transverse direction and
which become more convex in a longitudinal direction and/or twists
in a longitudinal direction.
[0043] The variation in the longitudinal direction allows the
heater mat to be produced with the laminate of the heater mat
having slight conformance to double-curved surfaces, such as are
found in a wing leading edge which has a degree of twist and taper
along the leading edge.
[0044] According to another aspect, there is provided a method of
manufacturing an electrothermal heater mat, comprising the steps
of: laying up a stack comprising substantially planar layers which
include a plurality of dielectric layers and a heater element
layer; wherein each layer comprises thermoplastic material; heating
the stack to a first temperature and laminating together the layers
of the stack so as to produce a preform; positioning the preform on
a convex surface of a tool; heating the preform to a second
temperature which is lower than the first temperature and is in a
range of 180.degree. C..+-.30.degree. C.; and forming the preform
around the convex surface of the tool. Features of the previous
aspect of the present invention may be applied mutatis mutandis to
this aspect, and vice versa.
[0045] According to another aspect, there is provided a method of
manufacturing an ice-protected apparatus, comprising the steps of:
performing a method of manufacturing an electrothermal heater mat
as described herein and, in the heating and forming steps,
positioning the preform against a component comprising
thermosetting material and using the heating of the heating step to
bond the preform to the component.
[0046] Thus, the heater mat may be co-bonded to, for example, an
epoxy component (such as part of an aerofoil leading edge) and
cured in a single heating operation. In other words, the heating
operation which is used to soften the preform to form the final
shape of the heater mat is also used to cure the component made of
thermosetting material.
[0047] In some examples, the component has a surface which has the
second configuration and to which the preform is bonded. For
example, the component may have a rear surface which is
concave.
[0048] In some examples, in the heating and forming steps, the
preform is positioned between a tool and the component. Thus, the
component helps to hold the final (second) shape of the heater mat
during the creep-forming operation. This may remove the need to use
a vacuum bag, e.g. if the preform is completely covered by the
component when the preform is seated on the tool.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] Some examples will now be described with reference to the
accompanying drawings in which: --
[0050] FIG. 1 is a diagrammatic plan view of an aircraft having
slats in the leading edge of a wing.
[0051] FIG. 2 is a diagrammatic perspective view of a nose skin of
a wing slat of FIG. 1.
[0052] FIGS. 3 to 9 depict in a diagrammatic manner the stages of a
manufacturing method for producing an electrothermal heater mat, in
accordance with a first example.
[0053] FIG. 10 shows a variation of the manufacturing method of the
first example.
[0054] FIG. 11 shows a variation of the tool used in the first
example.
[0055] FIG. 12 show a further variation of the tool used in the
first example.
[0056] While the invention is susceptible to various modifications
and alternative forms, some examples are shown by way of example in
the drawings and are herein described in detail. It should be
understood, however, that the drawings and detailed description of
these examples are not intended to limit the invention to the
particular forms disclosed. In addition although individual
examples may have been discussed, the invention is intended to
cover combinations of those examples. The invention covers all
modifications, equivalents and alternatives falling within the
spirit and the scope of the present invention as defined by the
appended claims.
DESCRIPTION
[0057] FIG. 1 is a plan view of an aircraft 1 having a wing 11
along the leading (forward) edge of which are positioned five wing
slats 12. Each wing slat 12 incorporates an electrothermal ice
protection system.
[0058] FIG. 2 is a diagrammatic perspective view of a demountable
nose skin 13 of one of the wing slats 12 of FIG. 1. The
configuration of the nose skin 13 may be generally the same as in
EP-A-1,757,519 (GKN Aerospace) which discloses a wing slat having a
demountable forward section comprising a nose skin.
[0059] The nose skin 13 comprises an erosion shield 14 and an
electrically-powered heater 2.
[0060] The heater 2 comprises a heater blanket or mat 3 and a
bundle of wires or lines 4 which connect the heater mat 3 to
associated power supply and control electronics.
[0061] The erosion shield 14 is generally rectangular and has a
front surface 141 which is convexly curved and a rear surface 142
which is concavely curved. An apex 1411 of the front surface 141
provides the leading edge of the aircraft wing 11.
[0062] The heater mat 3 is generally rectangular and has a front
surface 31 which is convexly curved and a rear surface 32 which is
concavely curved. The convex front surface 31 conforms to the shape
of and is bonded to the rear surface 142 of the erosion shield 14.
In this way, thermal energy generated as the heater mat 3 is
operated passes, by conduction, into the erosion shield 14 in order
to provide an ice protection function. The erosion shield 14 is
metallic and may be made of aluminium or titanium. A function of
the erosion shield 14 is to protect the aircraft against lightning
strikes by absorbing and dissipating the lightning current.
[0063] The concave rear surface 32 of the heater mat 3 may be
attached to a support structure of the wing slat 12.
[0064] A manufacturing method for producing an electrothermal
heater mat, in accordance with a first example of the present
invention, will now be described with reference to FIGS. 3-9 which
depict, in a diagrammatic manner, the stages of the method.
[0065] FIG. 3 is a diagrammatic representation of a stack 500 of
dielectric layers 51-56 at a first stage of an example
manufacturing method.
[0066] The depiction of the dielectric layers 51-56 is diagrammatic
and does not depict scale or proportions. For example, in relation
to each dielectric layer, the thickness has been exaggerated for
the sake of clarity. Also, the width and length of the layer have
been reduced for the sake of clarity. In a practical example, the
dielectric layer would be generally rectangular and would be a
sheet having a width ranging typically from 0.25 m (meter) to 0.6 m
and a length ranging from typically 1 m to 4 m. In one example in
use, the width of the sheet may wrap around the chord at the
leading edge of the wing, and the length of the sheet may extend
along the span of the wing. The dielectric sheet (the dielectric
layer) would also typically have a thickness of 0.05 mm
(millimeter) to 0.25 mm.
[0067] The dielectric layers 51-56 are made from a high-temperature
engineering thermoplastic (for layers 51, 52, 55 and 56) or from a
reinforcement material (such as glass fibres) which is impregnated
with the high-temperature engineering thermoplastic (for layers 53
and 54).
[0068] From the class of high-temperature engineering
thermoplastics, we currently use: PEEK (polyether ether ketone),
PEKK (polyetherketoneketone), PPS (polyphenylene sulphide), PEI
(polyetherimide) or PES (polyethersulphone) or mixtures thereof.
These materials have been selected based on the requirement for a
suitable glass transition temperature and suitable thermal fatigue
performance PEEK and PEKK are particularly preferred because PEEK
has the necessary mechanical performance and is particularly
receptive to a flame sprayed metal coating, and PEKK has similar
properties but is easier to bond to the metal material.
[0069] The central or core dielectric layers 53, 54 which will lie
on the neutral axis NA during the forming operation (see FIG. 7)
are reinforced, and they constitute a central group 501 of the
layers.
[0070] The dielectric layers 51, 52 which are positioned outward of
(above) the central group 501 are not reinforced, and the layers
51, 52 constitute an upper (outer) group 502 of the layers.
[0071] The dielectric layers 55, 56 which are positioned outward of
(below) the central group 501 are not reinforced, and the layers
55, 56 constitute a lower (outer) group 503 of the layers.
[0072] FIG. 3 shows only six dielectric layers, but additional
dielectric layers may be included. The central group 501 is shown
as comprising two reinforced layers, but additional reinforced
layers may be included in the group, or the group could be reduced
down to a single layer.
[0073] The reinforcement is preferably woven e-glass
(electrical-grade glass). For example, 48 gsm plain-weave e-glass
may be used, with a coating compatible with thermoplastic materials
and processing temperatures (the coating promotes adhesion between
the reinforcement and matrix, improving mechanical properties).
[0074] A heater element has been flame sprayed on the dielectric
layer 53, and a temperature sensor has been flame sprayed on the
dielectric layer 54. A conductive ground plane has been flame
sprayed on the dielectric layer 55. This flame spraying may be done
in the manner described in GB-A-2,477,336, GB-A-2,477,337,
GB-A-2,477,338, GB-A-2,477,339, and GB-A-2,477,340 (all in the name
of GKN Aerospace).
[0075] The stack 500 may be layed up on a flat, lower platen 61
(see FIG. 4). In the next stage of the manufacturing process, an
upper platen 62 is brought into contact with the top of the stack.
The platens 61, 62 are heated, and heat and pressure are applied by
the platens 61, 62 to the stack 500 so that the dielectric layers
are laminated together. The temperature applied to the stack 500 is
typically around 400.degree. C., which is above the melting point
of the thermoplastic material of the dielectric layers 51-56. The
result (shown in FIG. 5.) is a laminated preform 7 which is an
intermediate product in the manufacturing process.
[0076] In the preform 7, the layers 51-56 have merged or fused
together so that the preform 7 may be considered to be monolithic
from a structural point of view. This is assisted when the
thermoplastic material of the layers 51-56 is PEEK or PEKK, because
these materials are particularly good at ensuring that the layers
will fuse or bond together to become structurally monolithic and
will not delaminate.
[0077] The platens 61, 62 are made of steel and are therefore
suited to the lamination temperature of around 400.degree. C.
[0078] As may be seen in FIG. 5, the preform 7 at this stage of the
manufacturing process is a flat (2-D) laminate. The preform 7
typically has a thickness of 1 mm or less.
[0079] The next stage of the process is shown in FIG. 6, which is a
diagrammatic perspective view of the preform 7 when positioned on a
forming tool 8 having an outer tool surface 81 which is convex
(generally U-shaped) in cross-section in the transverse direction
82. The tool 8 is provided with positioning pins 83 spaced apart in
the longitudinal direction 84 along the ridge or apex 85 of the
tool 8.
[0080] The tool 8 is diagrammatically drawn and, in practice, we
include a strengthening frame inside the outer tool surface 81. The
tool 8 will be subjected to temperatures in the order of
180.degree. C..+-.30.degree. C. and may therefore be made of
aluminium, or at least the tool surface 81 which directly supports
the preform 7 may be made of aluminium.
[0081] The purpose of the tool 8 is to change the configuration of
the preform 7 from the first configuration (the planar or flat
(2-D) configuration) of FIG. 5 to a desired final or second
configuration shown in solid line in FIG. 9 in which the formed
preform 7 is now a shaped heater mat 3 which is has a non-planar or
curved (3-D) configuration.
[0082] When the preform 7 is formed on the tool surface 81 of the
tool 8, the preform 7 will initially adopt the shape of the tool
surface 81, and, when the preform 7 (the heater mat 3) is removed
from the tool 8, it will spring back or expand outwards to some
extent, with the amount of spring-back being dependent on factors
such the duration and temperature of the forming operation.
[0083] The shape initially adopted by the preform 7 is shown in
dashed line in FIG. 9 and is labelled 71. This shape may be thought
of as being an intermediate (third) configuration adopted by the
preform/heater mat, before it relaxes to the final (second)
configuration shown in solid line.
[0084] The positioning pins 83 extend into apertures of
complementary shape in the preform 7 in order to fix the relative
positioning of the preform 7 on the tool 8. The apertures in the
preform 7 are located along a longitudinal central axis of the
preform 7 which is aligned with the longitudinal ridge direction
84. Thus, a first half 72 of the preform 7 will be formed onto a
left-hand part 86 of the outer tool surface 81, and a second half
73 of the preform 7 will be formed onto a right-hand part 87 of the
outer tool surface 81.
[0085] The next stage of the manufacturing process is shown in
FIGS. 7 and 8 which are diagrammatic end views of the preform 7 and
the tool 8.
[0086] An autoclave or the like is used to heat the preform 7 and
the tool 8 to a temperature of about 180.degree. C. for a period of
about 30 minutes. The preform 7 is covered with a vacuum bag 9 and
air is removed from the space 91 covered by the vacuum bag 9. The
temperature of 180.degree. C. is above the glass-transition point
of 140.degree. C. of the thermoplastic material of the dielectric
layers 51-56, and thus the preform 7 undergoes a type of creep
deformation and is formed down onto the outer tool surface 81. In
the transition from FIG. 7 to FIG. 8, the preform 7 changes from
the first, 2-D (planar) configuration to a non-planar, 3-D
configuration which is the intermediate configuration 71.
[0087] In general, any form of applying heat and shape conformance
may be used. For example, an oven with a vacuum bag may be used, or
a heated press with appropriate pairs of matched-shape tooling may
be used.
[0088] Because the reinforcement in the preform 7 is concentrated
at the neutral axis NA (where the dielectric layers 53, 54
generally at the neutral axis are neither stretched nor compressed
as the forming occurs), the forming operation can be accomplished
at the relatively low temperature that is used (the typical
temperature of 180.degree. C..+-.30.degree. C., or .+-.20.degree.
C., or .+-.10.degree. C.) and thus, in turn, the tool 8 does not
have to withstand the higher temperatures (e.g. at least
340.degree. C.) that were previously typically used when forming a
2-D thermoplastic laminate into a 3-D shape. Thus, the tool surface
81 and much of the tool 8 may be made of aluminium instead of
steel.
[0089] The absence of reinforcement in the outer dielectric layers
51, 52, 55, 56 helps to reduce brittleness and to provide stability
during the forming process.
[0090] After 30 minutes or so, the preform 71 may be returned to
ambient temperature, and the vacuum bag 9 may be removed. The
preform 71 undergoes some spring-back and ends up in its final
(second) configuration shown in solid line in FIG. 9 as the heater
mat 3.
[0091] In other words, during the forming operation, the preform 7
is "over formed" (i.e., given more curvature in cross-section than
is ultimately desired), and after the forming operation the preform
7 springs back to the final (slightly less curved) shape.
[0092] An important benefit of the relatively low temperature used
in the forming operation (wherein the thermoplastic matrix material
is only softened and is not melted) is that the functional
electrical components of the preform/heater mat (such as the heater
element, the temperature sensor and the conductive ground plane)
are not impaired by the heating associated with the forming
operation.
[0093] We have conducted trials to characterise the change in
electrical characteristics of the functional components of the
heater mat as a consequence of the heating applied to the
preform/heater mat in the forming operation, and the trials have
confirmed that there is only a minimal, repeatable resistance
shift.
[0094] These trials have been undertaken by forming heating
elements around the radius of a curvature, such that the long edge
of the heating element undergoes maximum geometric strain. Separate
experiments were conducted to establish the effect of varying both
the width and thickness of heating elements (Table 1 and Table 2).
All trials demonstrated a change in electrical resistance of no
more than 2% as a consequence of forming. This is considered a
small and repeatable result in the wider context of a heater mat
manufacturing process.
[0095] In the trials, the electrical resistance of the heating
element was measured pre-and-post forming with a meter of an
accuracy suitable to the test.
TABLE-US-00001 TABLE 1 Resistance Shift of Various Heating Element
Thicknesses as a Consequence of Forming Sample Sample Sample Sample
Sample Set 1 Set 2 Set 3 Set 4 Set 5 Heating Element Thinnest Thin
Mid- Thick Thickest Thickness Range Average Resistance 0.42% 1.26%
0.22% 0.42% 0.15% Shift Post-Forming
TABLE-US-00002 TABLE 2 Resistance Shift of Various Heating Element
Widths as a Consequence of Forming Sample Sample Sample Sample
Sample Sample Sample Sample Sample Set 1 Set 2 Set 3 Set 4 Set 5
Set 6 Set 7 Set 8 Set 9 Element Width 2 mm 3 mm 4 mm 5 mm 6 mm 8 mm
10 mm 12 mm 14 mm Average Resistance Shift 0.74% 0.89% 0.68% 0.67%
0.84% 1.00% 0.98% 1.04% 1.19% Post-Forming
[0096] A variation of the manufacturing process is shown in FIG.
10. The vacuum bag 9 is not used in this variation. Instead, a
component 15 made of thermosetting material is placed on top of the
preform 7. The component 15 already has the second configuration.
The component 15 has a convex outer (front) surface 151 and a
concave inner (rear) surface 152. The concave inner surface 152 is
placed on the top surface 74 of the preform 7 and sandwiches the
preform against the tool 8. The temperature of the forming
operation (the temperature of 180.degree. C..+-.30.degree. C., or
.+-.20.degree. C., or .+-.10.degree. C.) serves to creep-form the
preform 7 and also to cure the thermosetting material of the
component 15 and to bond the preform 7 to the component 15.
[0097] FIG. 11 shows a tool 8A which is a variation of the tool 8
of FIG. 6. The tool 8A varies by tapering in the longitudinal
direction 84 by becoming more curved (more convex) in the
longitudinal direction. As may be seen in FIG. 11, the dimension X1
is greater than the dimension X2.
[0098] FIG. 12 shows a tool 8B which is another variation of the
tool 8 of FIG. 6. Compared with the tool 8, the tool 8B twists in
the longitudinal direction 84 of tool 8. Tool 8B is shown as
twisting to the right as viewed in FIG. 12. Also, with tool 8B, the
ridge 85 is not level (as with tool 8) and instead the ridge
reduces in height by the dimension X3.
[0099] The variations of shape embodied in tools 8A, 8B are
visually exaggerated in FIGS. 11 and 12, but the variations may be
used in practice to impart some slight degree of taper, twist and
complex curvature to the preform 7 as it is formed into the final
(second) configuration desired for the heater mat 3.
* * * * *